1. Technical Field
The subject matter described herein relates to a device for controlling the flow of light and radiant heat through selective reflection. The technology has particular, but not exclusive, application in passive or active light-regulating and temperature-regulating films, materials and devices, especially as construction materials.
2. Description of the Related Art
The problem of controlling the flow of radiant energy, e.g., light and heat, in particular in applications such as regulating solar heat gain in buildings and in other applications has previously been addressed using many optical methodologies. Photodarkening materials have been used for decades, for example, in sunglass lenses, to selectively attenuate incoming light when stimulated by ultraviolet (UV) radiation. When incorporated into windows, such materials can be used to regulate the internal temperature of a structure by darkening to attenuate bright sunlight, and by becoming transparent again to allow artificial light or diffuse daylight to pass through unimpeded. Such systems are passive and self-regulating, requiring no external signal other than ambient UV light in order to operate. However, because they are controlled by UV light rather than by temperature, such systems are of limited utility in temperature-regulating applications. For example, they may block wanted sunlight in cold weather as well as unwanted sunlight in hot weather.
Electrodarkening materials have also been used to regulate the transmission of light. The most widely used electrodarkening material is a liquid crystal sandwiched between two highly efficient absorbing polarizers, which attenuate slightly more than 50% of the light passing through them, primarily by absorption. This material is controlled by an electric field created by coatings of a transparent, electrically conductive material such as indium-tin-oxide (ITO). These liquid crystal panels are typically used in video displays, which are designed to not be isotropic under operating conditions and have seen only very limited use in building materials. This is, in part, because of the significant infrastructure required to utilize them, including electrical wiring and power sources, and the requirement of either sophisticated control systems, sensors, and algorithms, or extensive user inputs, to set the state of the materials and thus regulate the light, heat, and radiant energy through them. Electrodarkening and photodarkening materials attenuate incoming light primarily through absorption rather than reflection, meaning they will heat up when exposed to bright light. The heat absorbed by these materials may also offset the reductions in radiative transmission, and thus place significant limits on their ability to regulate temperature.
Wire-grid polarizers (WGPs) which reflect infrared light rather than absorbing it, have been used since the 1960s and are described for example in U.S. Pat. No. 4,512,638 to Sriram, et al. With the advent of nanoscale lithography in the 1990s and 2000s, it became possible, though expensive, to produce broadband, wire-grid polarizers that reflect in visible and ultraviolet wavelengths, for use with high-end optics and laser technology as described, for example, in U.S. Pat. No. 6,122,103 to Perkins, et al.
More recently, low-cost reflective polarizer films combining the properties of a layered-polymer distributed Bragg reflector (DBR) with a stretched-polymer polarizer have been introduced. Such reflective polarizers are used in video displays to enhance brightness by reflecting the attenuated light back into the device rather than absorbing it as described, for example, in U.S. Pat. No. 7,038,745 to Weber, et al. and U.S. Pat. No. 6,099,758 to Verrall, et al. Such reflective polarizers can exhibit specular reflection for one polarization of light, as in a mirror, or diffuse reflection for one polarization of light, as in a coating of white paint, or a combination of the two. These films were developed specifically for the video display market and have not been used outside of it.
In addition, reflective polarizers can be made from certain types of liquid crystals. Whereas wire-grid polarizers and stretched polymer polarizers are linearly polarizing, these liquid crystal polarizers (LCPs) are generally circularly polarizing. Thus, light of one helicity (i.e., right- or left-handed) is transmitted and light of the opposite helicity is reflected.
Thermal switches allow the passage of heat energy in their ON or closed state, but prevent it in their OFF or open state. These switches are mechanical relays, which rely on contact between two conducting surfaces (typically made of metal) to enable the passage of heat. When the two surfaces are withdrawn, heat energy is unable to conduct between them except through the air gap. If the device is placed in vacuum, heat conduction is prevented entirely in the open state. Another type of thermal switch involves pumping a gas or liquid into or out of a chamber. When the chamber is full, it conducts heat. When empty, there is no conduction, although radiative transfer across the chamber may still occur.
Light can be blocked by optical filters which absorb or reflect certain frequencies of light while allowing others to pass through, thus acting like an optical switch. Also, the addition of a mechanical shutter can turn an otherwise transparent material-including a filter-into an optical switch. When the shutter is open, light passes through easily. When the shutter is closed, no light passes. If the mechanical shutter is replaced with an electrodarkening material such as a liquid crystal, then the switch is “nearly solid state,” with no moving parts except photons, electrons, and the liquid crystal molecules themselves. Other electrodarkening materials, described for example in U.S. Pat. No. 7,099,062 to Azens, et al., can serve a similar function. These optical filter/switch combinations are not passive, but must be operated by external signals, e.g., electrical signals.
Switchable mirrors are based on reversible metal hydride and metal lithide chemistry, described for example in U.S. Pat. No. 7,042,615 to Richardson. These switchable mirrors rely on the physical migration of ions across a barrier under the influence of an electric field and therefore have limited switching speeds and cycle lifetimes. In addition, electrically operated “light valves” combine liquid crystals with one or more reflective polarizers as described, for example, in U.S. Pat. No. 6,486,997 to Bruzzone, et al. In these devices, the liquid crystal typically serves as an electrotropic depolarizer, i.e., as a structure that changes or switches the rotation of the polarity of the light that passes through it on and off under the influence of an electric field. Some of these devices may be thought of as switchable mirrors, although they are rarely described that way, since their primary application is in video displays and advanced optics.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.